In the motion picture industry, there is an increasing shift from the use of photographic film as the picture carrying medium to the use of magnetic tape (videotape). Even when material is initially recorded on photographic film, and/or the final prints are to be on photographic film, there are time and cost advantages in being able to use videotape during the editing process. The use of videotape does however have some disadvantages so far as the editing process is concerned, particularly during sound editing. The conventional mode of editing sound when the picture information is on photographic film is to use a mechanical synchronizer in which the picture film and a film or films carrying the sound track or tracks (the term film is used for the carrier medium whether the sound is recorded optically or magnetically) can be advanced or moved backwards in unison, for example by means of a common drive shaft with ganged sprockets, as in the Moviola (trade mark) synchronizer and flat bed type synchronizers. The operator adjusts the relationship of the various films by lifting them out of engagement with the drive shaft sprockets and moving them longitudinally. This mode of operation is dependent on the ability to pass the picture film over the shaft together with the sound films so that the picture information is presented to the operator in a manner precisely related to the position of the synchronizer shaft. This is not of course practical with videotape, upon which the picture information is not directly visible to the operator and which is not adapted for the positive mechanical drive permitted by the sprocket holes in conventional film. This has meant that entirely different editing techniques have been required for editing videotape sound, and editors accustomed to the use of mechanical synchronizers have found it difficult to adapt to such techniques. Moreover, it has remained difficult to achieve accurate editing of sound films in conjunction with videotaped picture information, particularly with the cheaper helical scan recorders, since cueing information conventionally recorded on longitudinal tracks on the tape cannot be recovered whilst the tape is stationary during display of a single frame, or the tape is being moved frame by frame, or at a low rate of speed, or backwards.
Proposals have been made to introduce coded information identifying individual frames into the vertical intervals of the video information recorded on videotape so that this information may be recovered even when a single frame is being displayed during playback of the tape. Such proposals are contained in U.S. Pat. Nos. 3,925,815 (Lemelson), 3,748,381 (Strobele et al) and 3,740,463 (Youngstrom et al) but although such systems are undoubtedly useful in locating and identifying specific frames on the tape, they do not provide a means by which editors can employ the sound editing techniques they are accustomed to use when the picture information is on photographic film.